1. Field of the Invention
The present invention is related to bicycles, and more particularly to gear shifting mechanisms for bicycles.
2. Description of the Related Art
The transferring of a drive chain from one sprocket to an adjacent sprocket has been the common practice on bicycles for the last half century. The conventional mechanism for doing the transferring is called a “derailleur.” Conventional derailleurs in commercial use today move laterally to transfer the chain laterally from one sprocket to another.
An early example of a derailleur is disclosed in Nagano, U.S. Pat. No. 3,974,707 for a Derailleur For A Bicycle which discloses a derailleur for a bicycle having a fitting member, two linkage members and a movable member, which are all movable with respect to each other. Between two of the members there is a device for positioning cages having chain-guide pulleys mounted on the movable member, whereby the derailleur enables the cages to be properly positioned and kept in the position at each speed-change stage of the bicycle so that an accurate and light speed-change operation may be performed.
Another example is disclosed in Nagano, et al, U.S. Pat. No. 4,241,617 for a Derailleur For A Bicycle which discloses a derailleur for a bicycle adapted to shift a driving chain to a selected one of two or more sprocket wheels by pushing or pulling a push-pull control wire.
Yet another example is Kojima, et al., U.S. Pat. No. 5,518,456 for a Bicycle Derailleur which discloses a bicycle derailleur operable by a control cable to shift a chain between a large gear and a small gear, and includes a parallelogram link mechanism having a base member connectable to a bicycle frame, a support member, and a pair of pivotal links extending between the support member and the base member, with a return spring for biasing the support member in one direction.
Yet another example is Shahana et al., U.S. Pat. No. 7,044,874 for a Bicycle Rear Derailleur which discloses a derailleur that includes a base member for attachment to a bicycle frame and a movable member supporting a chain guide such that the chain guide moves laterally to shift a chain among a plurality of gears. The base member has a laterally inner surface and a laterally outer surface, and it includes a projection extending from one of the laterally inner surface and the laterally outer surface for engaging a corresponding recess in the frame.
It is generally a complex matter to move the chain guiding surfaces or pulleys laterally while maintaining alignment of the guiding surfaces to the chain line for smooth running between shifts. The conventional solution to this problem is to use a precision parallelogram so that lateral movement can be accomplished with substantially pure translation with no rotation.
In the case of the rear derailleur, the precision parallelogram is also burdened with the additional task of supporting a chain tensioner to take up slack in the chain. There is more or less chain slack depending on which front and rear sprockets are selected to produce a drive ratio.
An additional deficiency of a parallelogram derailleur especially the rear derailleur is that the parallelogram needs room to operate. Generally the parallelogram of the rear derailleur swings outboard of the sprockets to which it is guiding the drive chain. This places the parallelogram outside the protective envelope of the chain stays for most of its operation, especially in the smaller or higher drive ratio rear sprockets.
A major problem for city and off road bicycles today is that this rear derailleur is in harm's way, swinging outside the chain stays. Its delicate precision parallelogram is thus easily and frequently damaged. The damage can occur when the bicycle is ridden in a rough environment with tree branches, roots and rocks. The damage can also occur when the bicycle is transported with a group of bicycles in the back of a truck or car. The damage even frequently occurs when a new bicycle is being shipped from the factory to a bicycle shop before it is offered for sale.
These problems are generally due to the awkward exposed location of the rear derailleur.
Another disadvantage of the common rear derailleur is that it requires a precise relationship to the sprockets in order to precisely guide the drive chain to a selected sprocket. But, in spite of this precise requirement, the derailleur is not mounted directly to the rear hub. It is instead bolted to the frame or “derailleur hanger”. The derailleur hanger is an element of a bicycle frame that is constantly being damaged due to impact forces transferred to it by the awkwardly located rear derailleur. Every frame designer has a novel way to either strengthen or make the hanger replaceable. These design variations complicate the precise relationship needed between the derailleur and the sprockets. This variation from frame to frame makes it necessary to include a range of motion limit adjustments on the derailleur so that each bicycle can be custom adjusted to restore the required relationship between the derailleur and sprockets. Much care must be taken to align the hanger on a new bicycle or after impact to the derailleur. Hangers often break off and render the whole bicycle frame useless.